A long standing goal of immunogenetics has been to understand the genetic basis of antibody diversity and responsiveness, the mechanisms through which the apparently limitless variety of antibody sequences are created. Our aim is to complete the determination of the structure, and the complete sequence, of the immunoglobulin heavy chain (Igh) locus in two strains of mice that have markedly different Igh gene complexes; we will complete the assembly of sequence-ready Bacterial Artificial Chromosome (BAG) contigs from C57BL/6 and 129/Sv; we will identify every variable region (Vh) gene with their 5' and 3' regulatory and RSS elements and compare these germline specificity repertoires. Comparison of the two mouse haplotypes will also reveal conserved noncoding sequence elements; these will likely include control elements used in the regulation of this complex locus, and we will investigate their possible roles in regulating the activation, expression, and replication of Igh. [unreadable] [unreadable] [unreadable]